Self-cleaning ovens, by design, must be capable of operation at higher temperatures than those associated with normal cooking. The self-cleaning process, known as pyrolytic cleaning, requires temperatures sufficiently high to incinerate food soils deposited or "baked on" the ovens' walls. Generally speaking, the shorter an oven's self-cleaning cycle--the period of time required to heat the oven to self-cleaning temperature, operate the oven at the self-cleaning temperature, and then return the oven to a normal, cooking temperature--the higher the oven temperature required for incineration.
Prior to the present invention, no gas-fueled oven for cooking has been developed which is capable of repeatedly self-cleaning at temperatures exceeding approximately 900 .degree. F. because of structural limitations of the oven. For the case of residential ovens, there has been little need to develop a self cleaning, gas fueled oven capable of cleaning at temperatures above 900.degree. F., since the approximately four hours long self-cleaning cycle typical of residential ovens which self-clean at temperatures near 900.degree. F. without a convection fan is short enough not to significantly affect the availability of the oven for household cooking purposes. For the case of commercial ovens, however, there has been a substantial need to develop an oven capable of cleaning at temperatures above 900.degree. F., since a self-cleaning cycle of four hours, or even two hours, is sufficiently long to cut into a restaurant's food production efficiency.
In view of the above, one object of the invention is to provide a self-cleaning, gas-fueled cooking oven which is capable of repeatedly self-cleaning at temperatures exceeding 900.degree. F.
Cost and durability have been the principal factors which have thwarted the development of a faster-cleaning gas-fueled oven. An oven with only the typical one-stage gas burner takes too long to reach temperatures exceeding 900.degree. F. Adding a second such burner, to be turned on in conjunction with the first when self-cleaning is desired, solves this problem, but creates a second problem. The second burner requires second, separate safety and ignition systems, increasing the cost of the oven to a point where it may exceed the savings gained by reducing the time the oven is taken out of productive use for self-cleaning.
If a single two-stage gas burner is used in place of the typical one-stage burner, with the first, lower output stage used for cooking and the second, higher output stage used for self-cleaning, the cost problem associated with a two burner system is avoided, but the problem of designing and fabricating a flameholder which will accommodate both output stages is presented.
The function of a flameholder is, for a given flow rate of a fuel mixture through a burner, to maintain combustion by holding the flame produced by the burner a relatively fixed distance away from the burner. Flameholders commonly use ports for maintaining safe combustion. A standard starting point for flameholder design is finding the quench diameter, the port diameter for which the flame is self-extinguishing. A self-extinguishing flame is one which does not propagate a sufficient distance back toward the burner to cause "flashback". Flashback, which can cause excessive heating on the burner and can thus ruin it, occurs when gas behind the flameholder is ignited. To prevent such ignition, the flame must not be allowed to settle so close to the flameholder that the flameholder is heated to combustion temperature. The quench diameter depends upon several factors, including the fuel being used, the ratio of air to fuel in the fuel mixture, and the velocity of the fuel mixture as it leaves the port; it can be calculated using formulas known to those skilled in the art.
Once the quench diameter is determined, the port area needed to accommodate the expected burner output is customarily calculated, again using formulas known to those skilled in the art. The port area is the cross-sectional area of the port perpendicular to the direction of flow through the port. The greater the expected burner output, the larger the port area generally required to prevent the flame from lifting off the burner. If the port area required exceeds the quench diameter, more than one port may be required.
If the burner for the flameholder being designed has two output stages (for example, the cooking stage is defined by a range of temperatures, while the cleaning stage is defined by a single temperature at a higher rate of consumption of gas) rather than the usual single output stage, the required port size can be determined by calculating the quench diameter for the lower output stage. The number of required ports of this size can then be determined by calculating the total port area needed to prevent lift-off. When the design is for a commercial gas oven with a desired maximum cooking temperature below 500.degree. F. and a desired self-cleaning temperature exceeding 900.degree. F., heretofore unsolved problems are presented. First, the number of ports required is very large, resulting in unattractive fabrication costs. Second, the multitude of similarly sized flames produced by these ports causes a type of oscillation known as "combustion driven oscillation," which generates intolerable noise. Third, the tops and bottoms of typical relatively long but shallow oven burner boxes cannot tolerate the heat directed upward and downward by common cylindrical flameholders.
Accordingly, another object of the invention is to provide a flameholder which can accommodate a single gas burner of two or more stages using fewer ports than typical prior multistage flameholders.
A further object of the invention is to provide a flameholder which can accommodate a single gas burner of two or more stages without producing intolerable noise.
Another object of the invention is to provide a flameholder which does not cause the top and bottom of a common gas oven burner box to overheat at temperatures exceeding 900.degree. F.
Another problem which must be addressed in designing a gas-fueled oven which self-cleans at temperatures exceeding 900.degree. F. is associated with the temperature differential which develops between the inner and outer portions of the oven door. Both portions are commonly composed of a panel with interconnected sidewalls. They are fit together like the top and bottom of a shoe box, with the outer portion, often called the "skin," overlapping the inner portion. Screws along the sidewalls are commonly used to hold the two portions together. When the temperature of the door is raised, the inner portion of the door expands relative to the overlapping outer portion, stressing the sidewalls. Repeated heating of the oven to temperatures exceeding 900.degree. F. causes the sidewalls to distort peripherally so that the door substantially loses its heat-sealing capability.
Accordingly, still another object of the invention is to provide a door for an oven which can withstand repeated heating of the oven to temperatures exceeding 900.degree. F.
Finally, a more general object of the present invention is to provide a self-cleaning oven having combined features which overcome or reduce the above-noted problems of the prior art.